1. Field of the Invention
The present invention relates to a digital signal processing circuit and an audio reproducing device using it such as a headphone device, an earphone device, and a speaker device.
2. Description of the Related Art
When music or the like is reproduced by supplying an audio signal to a speaker that is located in front of a listener, an acoustic image is located in front of the listener. Even for the same audio signal, its reproduction acoustic image is located inside the head of a listener when reproduction is performed by supplying the audio signal to a headphone; the location of the acoustic image is very unnatural.
In view of the above, a headphone device has been developed that locates the acoustic image of an audio signal outside the head.
FIG. 1 shows an example of such a headphone device. An analog audio signal SA is supplied to an A/D converter circuit 2 via an input terminal 1 and thereby A/D-converted to a digital audio signal SD. The digital audio signal SD is supplied to digital signal processing circuits 3L and 3R, which perform processing for locating an acoustic image outside the head.
For example, as shown in FIG. 2, in a case where a sound source SP is located in front of a listener M, a sound that is output from the sound source SP reaches the left ear and the right ear of the listener M along paths represented by transfer functions HL and HR, respectively.
The digital signal processing circuits 3L and 3R calculate the convolutions of the signal SD and impulse responses obtained by converting the transfer functions HL and HR to time domain signals, respectively. The impulse response can be determined in advance by measurement or calculation.
Signals SFL and SFR obtained by the signal processing are supplied to D/A converter circuits 4L and 4R and thereby D/A-converted to analog audio signals SL and SR, which are supplied to left and right acoustic units (electro-acoustic conversion elements) 6L and 6R of a headphone 6 via headphone amplifiers 5L and 5R, respectively.
Therefore, when the listener M listens to a reproduction sound of the headphone 6 mounted on his head, the reproduction sound of the headphone 6 is equivalent to a reproduction sound of a signal that has traveled along the paths having the transfer functions HL and HR and hence a state that the acoustic image SP of the reproduction sound is located outside the head of the listener M as shown in FIG. 2 is realized.
To realize the transfer functions HL and HR, each of the digital signal processing circuits 3L and 3R is given a FIR filter configuration as shown in FIG. 3, for example. As shown in FIG. 3, the signal SD that is output from the A/D converter circuit 2 is supplied to a plurality of cascade-connected delay circuits 3D via an input terminal 31. Signals that are output from the input terminal 31 and the delay circuits 3D are supplied to respective multiplication circuits 3M, and their multiplication outputs are added together by addition circuits 3A and a resulting signal is output from an output terminal 37.
Each of the delay circuits 3D gives a delay of one sampling period (one unit time) xcfx84 to the digital audio signal SD. Each of the multiplication circuits 3M has, as a coefficient, an impulse response at each sampling time that is obtained by converting the transfer function HL or HR to a time domain signal.
In reverberation room spaces such as a general room and a concert hall, reflection sounds and reverberation sounds occur in addition to a direct sound from a sound source and hence there is a tendency that a relatively long time is needed for impulse responses of those kinds to attenuate sufficiently.
In view of the above, the digital signal processing circuits 3L and 3R shown in FIG. 3 are required to have a large number of taps and hence many, for example, 1,024, pairs of delay circuits 3D and multiplication circuits 3M are needed.
As a result, for example, in a configuration in which the digital signal processing circuits 3L and 3R are formed by a DSP, a large-capacity memory is needed for the delay circuits 3D and hence the IC scale becomes large and the cost increases greatly. Further, since a large number of process steps are needed to implement many multiplication circuits 3M, a fast calculation process is required. The cost also increases in this respect.
The present invention has been made to solve the above problems in the art, and an object of the invention is therefore to decrease the number of taps of a digital filter used in an audio reproducing device that locates an acoustic image outside the head.
To attain the above object, the invention provides a digital signal processing circuit comprising two digital filters for reproducing impulse responses that represent transmission characteristics of a first path from a sound source to a left ear of a listener and a second path from the sound source to a right ear of the listener, respectively, the two digital filters producing output audio signals representing convolutions of an input audio signal and impulse responses obtained by converting transfer functions of the first and second paths to time domain signals, respectively, to thereby allow reproduction of reproduction sounds reflecting the transfer functions of the first and second paths, respectively; and a headphone for receiving the output audio signals produced by the two digital filters, respectively, to thereby locate a reproduction acoustic image of the input audio signal outside a head of the listener, wherein a sampling rate in each of the two digital filters is changed depending on a section of a response time of the associated impulse response.
With the above configuration, the number of taps of each digital filter is reduced, resulting in reduction in circuit scale.